The invention relates more particularly to a parking or emergency brake device, particularly for a motor vehicle, of the type comprising a controlled actuating device which exerts a determined force on a brake operating member in order to apply the said brake blocking means so as to immobilize the vehicle and to a method for operating such an actuating device.
Numerous motor vehicle braking systems are known, the function of which is technically either to reduce or cancel the speed of the vehicle so fitted, or to keep it stationary, particularly when the vehicle is at a standstill for a prolonged period of time.
Thus, for a vehicle braking system, a distinction is made between a first function, known as the service brake function, and a second function known as the parking or emergency brake function.
The braking system therefore comprises at least one brake mechanism intended to fulfill the main service brake function and/or the parking brake function, the braking system for this purpose comprising a service brake device and a parking brake device.
Brake mechanisms of the state of the art essentially consist of two broad families, namely disk brakes on the one hand, and drum brakes on the other, the respective workings of which brakes are well known and will not be recalled in detail in the remainder of the description.
These brake mechanisms usually comprise unique blocking means which are able to be actuated selectively either by the main service brake device or by the parking brake device because these devices are not actuated simultaneously and fulfill very distinct braking functions, unless the parking brake is being used as an emergency brake.
Specifically, the parking brake device may constitute an emergency brake that can be operated by the driver in the event of failure of the motor vehicle main braking device.
In order in particular to allow the parking brake device to fulfill this emergency brake function, the braking system comprises separate control means for each of the two braking devices of the brake mechanism.
Document FR-A-2.829.543 describes one example of a disk-brake mechanism in which the brake cylinder comprises, in the conventional way, hydraulic actuating means intended, in response to a command from the brake pedal of the vehicle, to fulfill the service brake function, and mechanical actuating means intended to fulfill the parking brake function.
More specifically, the mechanical means of actuating the parking brake comprise a combination of moving parts which are controlled by a lever forming an operating member on which the end of the cable of a parking brake actuating device exerts a tensile force so as to cause the pads that form the brake blocking means to be applied to the brake disk.
Document FR-A-2.697.599 describes an example of a drum brake mechanism comprising, in the conventional way, main hydraulic actuating means intended, in response to a command via the brake pedal of the vehicle, to fulfill the service brake function and auxiliary mechanical actuating means fulfilling the parking or emergency brake function.
More specifically, the auxiliary mechanical means of actuating the parking brake comprise an actuating lever, mounted so that it can pivot, and which is able, under the action of a tensile force exerted via an operating cable on one of its ends, to cause primary and secondary shoes that form blocking means bearing friction linings to move apart against the internal cylindrical surface of the brake drum.
Also known are hybrid brake mechanisms which combine use of a disk brake and of a drum brake in order respectively to fulfill the main brake function and the parking or emergency brake function, each brake comprising blocking means actuated by a respective conventional actuating device.
More specifically, a brake mechanism such as this comprises a hydraulic device for actuating the service disk brake and a mechanical device for actuating the parking drum brake.
Documents U.S. Pat. No. 4,854,423 or WO-A-96.41085 describe such brake mechanisms, otherwise known as drum-in-hat mechanisms because such mechanisms have a central brake drum surrounded by the brake disk extending radially outwards so that in axial section it has the overall shape of a hat.
As will have been understood, whatever the type of brake mechanism, the applying of a parking braking force to apply the brake blocking means is obtained by actuating a parking brake device comprising an operating cable which exerts a determined force, generally a tensile force, on a lever that forms an operating member in order to change the status of the brake and allow the vehicle to be immobilized.
The parking brake device for this purpose comprises a controlled actuating device of mechanical type, such as a handbrake which nowadays is commonly fitted to motor vehicles.
Such a mechanical actuating device usually comprises a manual operating member consisting of a handbrake lever which is arranged in the cabin and is secured to one of the ends of an operating cable, the other end of which actuates the parking brake operating member.
Thus, to actuate the operating cable secured to the handbrake lever, the driver exerts on the said lever a control force the purpose of which is to cause a determined force, generally a tensile force, to be applied to a lever that forms the member for operating the parking brake.
In the case of mechanical actuation devices for actuating the parking brake, the energy needed to produce the braking force or input is therefore supplied solely by the physical strength of the driver.
This is one of the reasons why mechanical devices of the handbrake type are nowadays in particular considered to be unpleasant and tiresome to use, especially for those who have little physical strength.
In addition, the braking force applied which causes the parking brake blocking means to be applied depends directly on the force applied by the driver, which means that if the driver exerts in sufficient force on the handbrake lever, or even forgets to actuate it at all, the vehicle will then not be correctly immobilized and this may lead to accidents.
It will therefore be understood that a parking brake implementing a mechanical actuating device of the handbrake type is not as reliable or as dependable as might be wished.
Other parking brake actuating devices have been developed, particularly actuating devices that can be actuated electrically or fluidically, typically hydraulically.
Thus, document U.S. Pat. No. 4,792,447 describes, for example, an electric device for actuating a disk brake mechanism intended to fulfill the parking brake function.
However, such an electric actuating device is not satisfactory either, particularly because it requires a significant array of planetary gears and a high-powered electric motor which leads to a prohibitive cost of manufacture and in addition raises the problem of reliability should the on-board power source that powers the said device fail.
Document EP-A-0.526.273 describes a disk brake equipped with a parking brake actuated hydraulically by a pressure exerted by the driver on the vehicle brake pedal that applies pads, forming blocking means, to the faces of the brake disk.
The way in which the device for hydraulic actuation of the parking brake works is for a force to be exerted first of all on the brake pedal to generate a control pressure which causes the pads to be applied to the disk with a given braking force, then, with the foot still sustaining the force on the pedal, for an electric switch to be operated to switch on an electric motor which drives, in rectilinear translation along the axis of the piston, a sleeve coaxial with the said piston so as to lock the piston in a parking braking position in which the vehicle is immobilized.
However, such a hydraulic actuating device has numerous disadvantages.
Specifically, it is down to the driver, as before with a mechanical device of the handbrake type, to generate, through his or her own physical strength, the hydraulic pressure needed to apply the parking braking force.
In addition, as the electric motor is unable to increase the force with which the pads are applied to the disk, there is a risk that the vehicle might not be correctly immobilized if the driver does not depress the pedal or does not press it hard enough.